[Support] Rename RoundUpToAlignment -> alignTo.

Rounding up an integer m to a nearest multiple of n where n is a power
of 2 is used very often if you are writing code to emit binary files.
RoundUpToAlignment is a small function to do that. But we found that the
function has a small but annoying issue; the name is a bit too long.
Because it is used quite often, that hurts readability.

This patch is to rename the function. The original name is kept as a
forwarder, so that submitting this patch won't immediately break Clang
and other LLVM projects. Once I update all occurrences of RoundUpToAlignment,
I'll remove the old name entirely.

http://reviews.llvm.org/D16162

llvm-svn: 257799